1. Field of the Invention
The present invention relates to an air/fuel ratio control system for an internal combustion engine
2. Description of Background Information
In an internal combustion engine provided with a three-way catalytic converter in the exhaust system, the air/fuel ratio of the mixture is controlled around a stoichiometric value (14.7:1 for example) in accordance with the composition of the exhaust gas and the operational conditions of the engine in order to provide an optimum operation of the three-way catalytic converter.
This type of air/fuel ratio control system is generally constructed to perform either one of an integral operation and a proportional-integration operation in accordance with an air/fuel ratio signal.
Further, it is known that in the case of air/fuel ratio control systems for an internal combustion engine equipped with the three-way catalytic converter, a higher efficiency of the purification of the exhaust gas is obtained when the deviation of the air/fuel ratio from the stoichiometric value is smaller and at the same time the period of inversion or turnover of the direction of the air/fuel ratio control, namely the control toward the rich side and the control toward the lean side, is shorter, or in other words, the frequency of the change in the direction of the air/fuel ratio control is higher.
However, in the case of the air/fuel ratio control system with the integration operation, it is difficult to shorten the period of the inversion of the direction of the air/fuel ratio conrol. This is because there is a relatively large delay of response during the change of the direction of the air/fuel ratio control after the actual air/fuel ratio has changed from lean to rich, or from rich to lean.